We have characterized a hyporeactivity factor produced in mice infected with murine CMV which inhibits induction of interferon of cell culture. This factor was separated from interferon by concanavalin-A column chromatography to which interferon did not bind. It, however, was neutralized by antiserum against mouse interferon. Two types of hyporeactivity factors were found following challenge of sensitized mice with BCG. One was indistinguishable from Type II immune interferon, while the other was neutralized by antiserum against regular interferon. Further characterization of these factors and their roles in producing hyporeactivity are under study.